Jack stand

ABSTRACT

A jack stand comprising a hollow column supported in an upright perpendicular position by a base plate at its lower end with a post telescopically supported within its upper end for extension relative thereto and a saddle at the upper end of the post. A pin mounted in inclined slots in the column by engagement with one of a plurality of notches on the post holds the latter at a predetermined position of extension. The pin is movable in the slots from an operative position supporting the post to an inoperative position to allow the post to be moved upwardly or downwardly relative to the column and back to an operative position in engagement with a different notch. There is a secondary support in the form of a plate at the top of the column movable into engagement with another notch and there is linkage interconnecting the pin and plate such that movement of the pin into and out of operative position is accompanied by movement of the plate into and out of operative position.

United States Patent [191 Binding Apr. 9, 1974 JACK STAND [75] Inventor: Kenneth W. Binding, Woburn,

' Mass.

[73] Assignee: Beatrice Foods Co., Chicago, Ill.

[22] Filed: July 19, 1972 [21] App]. No.: 273,182

248/354 S, 409; 254/DIG. 1, DIG. 4; 74/577 M, 527; 287/58 CT; 254/108, 109, 110, 111

[56] References Cited 'UNITED STATES PATENTS 265,034 9/1882 De Weese 74/577 M 3,688,439 9/1972 1 Doxsee l 49/49 X 2,730,903 l/1956 Trautman et al. 254/110 X 1,798,879 3/1931 Gormley 234/108 3,137,512 6/1964 Carpezzi 248/408 UX 486.822 l/l892 Chandler 248/407 278,617 5/1883 Smith 287/58 CT X 1,406,881 2/1922 Lorenzen 248/352 X 2,193,424 3/1940 Johnson 254/D1G. 1

FOREIGN PATENTS 0R APPLICATIONS 895,079 7/1949 Germany 74/527 7,303 6/1891 Great Britain 254/108 Primary Examiner--Roy D. Frazier Assistant Examiner-Rodney H. Bonck Attorney, Agent, or Firm-Robert T. Gammons,

[5 7] ABSTRACT A jack stand comprising a hollow column supported in an upright perpendicular position by a base plate at its lower end with a post telescopically supported within its upper end for extension relative thereto and a saddle at the upper end of the post. A pin mounted in inclined slots in the column by engagement with one of a plurality of notches on the post holds the latter at a predetermined position of extension. The pin is movable in the slots from an operative position supporting the post to an inoperative position to allow the post to be moved upwardly or downwardly relative to the col umn and back to an operative position in engagement with a different notch. There is a secondary support in the form of a plate at the top of the column movable into engagement with another notch and there is linkage interconnecting the pin and plate such that move ment of the pin into and out of operative position is accompanied by movement of the plate into and out of operative position.

12 Claims, 10 Drawing Figures FAIENTEUAPR 1914 SL802 658 sum 30F a FIGIOi JACK STAND BACKGROUND OF THE INVENTION The conventional jack stand has only one supporting element which is usually in the form of a pin inserted through aligned holes in the jack column and the post extension. When using the stand the body to be supported is raised with a lift jack of a mechanical or hydraulic kind whereupon the jack stand is placed below the body, the post raised to the approximate level of the body to align the nearest holes and the locking pin is inserted through the holes to lock the post in its extended position. This can be dangerous because the lift jack may fail before the jack stand is properly positioned and fixed. A supporting element may be embodied in the jack stand in such a way as not to require such manipulation; however, there remains the necessity of moving the post from one level to another to accommodate it to the desired height which in turn requires transfer of the supporting pin from one notch to another and so there is greater danger that the transfer may not be completely effected. Safety rules require that with this kind of jack an auxiliary supporting element be employed in the form of a pin adapted to be inserted through holes in the supporting column and post and so is objectionable for the reasons outlined above. It is the purpose of this invention to provide a jack stand which will not require extensive manipulation to place it in operative position, which embodies coincidentally operable primary and secondary supports so that should one fail the other will carry the entire load. Otherobjects are to provide a jack stand which is of relatively simple construction, which embodies a high degree of stability, which will accommodate itself to a canted load, which is adapted to be folded for ease of storage and is designed to be easily assembled both tofacilitate manufacture and use.

SUMMARY As herein illustrated, the jack stand comprises a rigid hollow column adapted to be supported in an upright position, a telescopically mounted post situated at the upper end of the column for extension relative thereto, said post containing lengthwise thereof a plurality of spaced notches, a first part supported by the column below the top thereof with which a notch in the post is adapted to be engaged to support the post in an extended position and a second part supported at the top of the column arranged to engage a notch in the post above the one notch, said second part being operative to hold the post extended in the event the first part fails. The first part is a pin mounted in inclined slots in the column transversely of the notched side of the post with which a notch is engaged to support the post in a predetermined position of extension. The pin is movable along the inclined slot to disengage it from the notch to permit the post to be moved up or down and the second part is a plate mounted at the top of the column which is movable into and out of operative position by movement of the pin in the slots from its operative to its inoperative position and vice versa. A spring operates to restore both the pin and the plate to their operative positions when the post is .moved to a position such that notches are situated in confronting relation to the pin and plate. There is a saddle mounted at the upper end of the post which is connected thereto axes perpendicular to the vertical axis of the column.

- A base plate is connected to the lower end of the col umn by means of a bolt such as to enable locking the column in' an upright position perpendicular to the base and when loosed to permit the column to be rotated to a prone position parallel to the base. There are protuberances on the base situated in a position to enter the lower end of the column when the latter is in its upright position to prevent displacement as long as the lower end is held engaged with the base and there are three triangularly arranged protuberances on the underside of the base situated at equal radial distances from the vertical axis of the column which provides three-point support for the stand. v

The invention will now be described in greater detail with reference to the accompanying drawings wherein:

FIG. 1 is-a side elevation of the jack stand; FIG. 2 is a front elevation of the jack stand; FIG. 3 is an enlarged section taken on the line 3--3 of FIG. 2 showing the supporting-post of the jack stand.

in its lowermost position and means supporting it in said position; r

FIG. 4 is a fragmentary section corresponding to FIG. 3. showing the post partially extended with the post supporting means disengaged from the post;

FIG. .5 is a section taken at right angles to FIG. 3 showingythe saddle at the top of the post tipped to one side; 7

FIG. 6 is a section at the lower end of the jack stand showing the supporting column resting on the base;

FIG. 7 is a section taken at right angles to FIG. 6 on the line 7-7 of FIG. 6;

FIG. 8 is a section corresponding to FIG. 6 showing the column raised from the base to enable tilting it to a prone position;

FIG. 9 is a section showing the column disposed in its prone position relative to the base; and

FIG. 10 is a plan view of the base.

Referring to the drawings (FIGS. 1 and 2), the jack stand comprises a column 10 supported at one end in an upright perpendicular position by a base member 12 with a post 14 telescopically mounted at its upper end for extension with respect to the column, the post having at its upper end a saddle member 16 upon which the part to be supported is adapted to rest.

The column 10 is a hollow rigid structure of substantially rectangular cross-section having spaced parallel side walls 18-18, a connecting rear wall 20 and a front wall comprising flanges 2222 which extend from the side walls part way across the front. The column is open at its upper and lower ends.

The base member l2 (FIG. 10) comprises a substantially flat plate 24 generally T-shaped in configuration having along the opposite sides of the leg portion spaced parallel, vertically disposed flanges 26-26 situated at a spacing such as to receive the lower end of the column 10 with the walls 18-18 engaged between the flanges 2626. The flanges 26-26 of the column contain aligned holes 28-28 and the walls 18-l8 of the column contain elongate, vertically disposed openings 3030. A bolt 32 having a head 34 at one end and threads 36 at the other end is disposed through the openings in the flanges and the openings at the lower end of the column and has mounted on it a wing nut 38 by means of which the lower end of the column may be locked between the flanges with its lower end engaged with the plate member 12. To prevent any possible displacement of the lower end of the column relative to the plate member 12 when locked the latter is provided with upwardly extending protrusions 40 and 42 of generally rectangular configuration which are adapted to project upwardly into the open end of the column when the latter is resting on the plate. These projections 40 and 42 may be formed by displacing the material of the plate 12 upwardly relative to its lower surface. The vertically elongate openings 30-30 permit the column to be raised relative to the clamping bolt 32 when the latter is loosed to disengage it from the protrusions 40 and 42 so that it may be rotated about its lower end and disposed in a prone position, as shown in FIG. 9, in engagement with the base plate 12.

To provide for stability the base plate 12 has at the opposite extremities of the head portion and at the distal end of the leg portion three downwardly extending protrusions 44 arranged triangularly and at equal radial distances from the vertical axis of the column to provide three-point support for the structure.

The post 14 is also of hollow construction comprising spaced parallel side walls 46-46 joined by a back wall 48 and is of a lesser cross-section than the column such that while the side walls 46-46 and back wall 48 engage the side walls 18-18 and the back wall of the column 10, the forward edges of the side walls 46-46 are spaced from and parallel to the flanges 22-22 at the front of the column. At the lower end of the post there is a guide plate 50 which correspondsin crosssection to the interior of the column so that all four edges engage the interior of the column.

The side walls of the column near the top contain transversely spaced inclined guide slots 52-52 in which there is mounted a pin 54 with its ends extending through the walls so as to be supported thereby and to be movable along the slots from the lower ends thereof to the upper ends thereof. Heads 55-55 at the opposite ends of the pin retain it'within the slots. The forward edges of the side walls 46-46 contain a plurality of transverse, longitudinally spaced notches 56-56 adapted to engage the pin 54 and by such engagement to support the post at a predetermined heightwise position relative to the column. The notches at their upper ends contain re-entrant recesses 58-58 corresponding in cross-section to the pin 54 so as to snugly receive the pin and to resist disengagement by vertical pressure applied in a downward direction. Inclined portions 60-60 cam the pin out of the notches when the post is drawn upwardly, that is, extended relative to the column. Lands 62-62 between the notches operate to hold the pin at the upper ends of the slots 52-52 while the post is being moved from one set of notches to the next and permits the pin to move back into the next lower set of notches when the latter confronts the pin. In order to lower the post the latter must first be raised sufficiently to take the pressure off the pin whereupon the pin is manually disengaged from the notches and held disengaged until the post is moved downwardly to the desired height or all the way down if the jack stand is to be removed.

Additional supporting means is employed in conjunction with the pin 54 comprising a flat plate 64 (FIGS. 1, 3 and 5) mounted at the top of the column for lateral movement thereon from a position in which its inner edge is disposed within a slot 56, as shown in FIGS. 1

and 3, to a withdrawn position as shown in FIG. 4. Movement of the supporting plate 64 into and out of operative position is effected simultaneously with movement of the pin 54 into and out of operative position. For this purpose there is fastened to the underside of the plate 64 a member 66 provided with downwardly extending long and shoft arms 68 and 70 containing aligned openings 72 and 74. A spindle 76 is fastened at one end to a plate 78 which in turn is secured at its ends to the flanges 22-22 in a horizontal position extending rearwardly from the plate 78 through the openings 72 and 74. The opening 72 is larger than the opening 74 and a spring 80 is mounted on the spindle, extends through the opening 72 and its ends are compressively supported in engagement, respectively, with the plate 78 and the arm 70. The arm 68 extends downwardly and has at its lower end a forwardly bent portion 82 which bears against the pin 54. As thus constructed, upward movement of the pin 54, produced by extension of the post, will move the arm 68 forwardly in opposition to the spring 80 which in turn moves the plate 64 forwardly so as to disengage its inner edge from the notch overlying it. When the pin 54 is release the spring 80 automatically turns both the plate 64 and the pin 54 I to their operative positions.

The saddle 16, as shown in FIGS. 3 and 5, is mounted at the top of the post on a pin 92 which extends through elongate vertically disposed slots -90 in the side walls at the upper ends of the posts and through openings 88-88 in downwardly extending flanges 86-86 at opposite sides of the saddle. The distance between the flanges 88-88 is greater than the width of the column so that the saddle is free to rock at right angles to the axis of the pin 92 to take up an inclination corresponding to the inclination of the part to be supported thereby and is provided with a protrusion 94 which normally holds it in a center position.

It should be understood that the present disclosure is for the purpose of illustration only and that this invention includes all modifications and equivalents falling within the scope of the appended claims.

I claim:

1. A jack stand comprising a rigid hollow column adapted to be supported at one end in an upright position, a post telescopically mounted at the upper end of the column for extension relative thereto, said column containing inclined guideways below its upper end, a first part supported in an operative position by the guideways, said first part being adapted to be displaced laterally in said guideways to permit extension of the post relative to the first part for re-engagement of another notch therewith, said first part being adapted to be returned to its undisplaced position when confronted by another notched portion of the post, a second part supported at the top of the column on its upper end, said second part being adapted in its operative position to have engagement with a notch at the top of the column when a notch lower down is engaged with the first part and to be displaced in an operative position of disengagement from the notch at the top of the post when the post is extended, means operable by lateral displacement of the first part to effect a coincidental lateral displacement of the second part, and spring means associated with said last means operable to return said first and second parts to their operative positions.

2. A jack stand comprising a hollow column adapted to be supported at one end in an upright position, a post telescopically mounted at the upper end of the column for extension relative thereto, said post containing at one side a plurality of notches spaced lengthwise thereof, transversely spaced inclined guideways on the column below the top, a bar mounted in the guideways transversely of the notched side of the post with which the notches in the post are adapted to be engaged to support the post in a predetermined position of extension, means on the post between notches operable by extension of the post to displace the bar laterally from the notch with which it is engaged, said bar being adapted to return to its undisplaced position when com fronted by a succeeding notch, a plate mounted at the top of the column adapted to be engaged with a notch at the top of the column'when a notch lower down is engaged with the bar, said plate being movable laterally from a position of engagement to a withdrawn position such as to permit the post to be moved relative to the column,'means interconnecting the plate and bar operable by movement of the bar to a lateral position to effect concomitant lateral movement of theplate to said withdrawn position, and spring means associated with the last means for returning the plate and bar to positions of engagement with the notches.

3. A jack stand according to claim 2, wherein there is means supporting the plate at the top of the column comprising a lug fixed to the underside of the plate containing a hole, a pin fixed at one end to the column and extending horizontally therefrom through said hole, and a spring mounted on the pin with its ends compressably engaged, respectively, with the column and with the lug.

4. A jack stand according to claim 2, wherein the plate rests on top of the column, a lug is fixed to the underside of the plate having spaced, downwardly extending arms containing aligned openings, a pin is fixed at one end to the column and extends horizontally therefrom through said openings, one of the arms being longer than the other and extending downwardly into adjacency with said pin, the openings in the longer arm being larger than that in the shorter arm and a spring mounted on the pin, said pin extending through the opening in the longer arm and being compressably engaged at its ends with the column and the shorter arm.

5. A jack stand comprising a rigid hollow column tension and when moved to a lateral position at the upper ends of the guideways to become disengaged from the notches to permit the post to be extended, means on the post between notches operable when the post is extended to move the pin to said lateral position in said guideways, said pin being adapted to be returned to the lower ends of the guideways when confronted by a succeeding notch, a flat plate mounted on the top of the column having an operable position in which it engages a notch at the top of the column when a notch lower down is engaged with said pin, and an inoperative withdrawn position when the pin is moved to said lateral position, an arm fixed to the plate having a part extending downwardly therefrom to a position adjacent the pin, said part being movable by the pin when the latter is moved laterally to effect simultaneous lat eral movement of the plate, and a spring engaged with the arm operable to return the plate to its operative position and simultaneously to return the pin to its operative position.

, 6. A jack stand comprising a rigid hollow column of rectangular cross-section adapted to be supported in an upright position, a telescopically mounted post of rectangular cross-section situated at the upper end of the column for extension relative thereto, said post being of a size such that three sides slidingly engage three sides of the column and the fourth. side is spaced from and parallel to the fourth side of the column, said post containing lengthwise thereof a plurality of spaced notches, a first part supported by the column below the top thereof with which a notchin the post is adapted to be engaged to support the post in an extended posi tion as determined by one of the notches engaged with the first part, a second part supported at the top of the column arranged to engagea notch in the post above the one notch, said second part being operated to hold the post extended in the event the first part should fail, and a guide member at the lower end of the post for slidably engaging all four sides of the column.

7. A jack stand according to claim 6, comprising a saddle plate mounted at the top of the post, said saddle plate containing a depression in its upper side and a corresponding protrusion on its lower side of a size to fit into the open top of the post, and means connecting the saddle plate to the post which permits rocking of the saddle plate about right angularly disposed axes perpendicular to the vertical axis of the post.

8. A jack stand according to claim 6 wherein one side of the post is spaced from and parallel to the corresponding side of the column and wherein the notches are situated along said one side and each notch inclines downwardly with respect to the perpendicular axis of the post and has at its upper end a re-entrant recess of a configuration corresponding to the cross-section of said part.

9. A jack stand according to claim 6, wherein said post has at its upper end aligned holes which are vertically elongate, comprising a saddle plate having spaced parallel, downwardly extending flanges containing aligned holes mounted on the post with the holes in the flanges in alignment with the holes in the post, and a pin'mounted in said holes.

10. A jack stand according to claim 9, wherein the saddle plate is provided 'with spaced parallel upwardly divergent flanges along the edges intermediate the downwardly extending flanges.

11. A jack stand according to claim 6, provided with a base member at the lower end of the column comprising a flat plate of larger area than the cross-section of the column on which the lower end'of the column rests, spaced parallel flanges rising from the plate in engagement with the opposite sides of the column at its lower end, said flanges and the lower end of the column containing openings for receiving a bolt, a bolt disposed in said opening, said bolt being adapted to lock the column in a perpendicular position with its lower end engaged with the plate, and said openings in the column being elongate so as to permit the column to be elevated with respect to the plate when the bolt is loose to enable rotating the column to a prone position, and means on the plate situated between the flanges in a position to enter the lower end of the column when the latter is situated in a perpendicular position operable to resist displacement of the lower end of the column so long as the lower end is held engaged with the plate.

12. A jack stand comprising slidingly interengaged parts, one of which is adapted to be supported at one end in an upright position and the other of which is adapted to be moved relative to the other end of the one part to effect extension or retraction, and means for locking the parts in a predetermined position relative to each other, comprising longitudinally spaced notches on one part, a pin mounted in inclined slots in the other part and a flat plate mounted on the other part above the pin at a distance therefrom corresponding to the distance between said notches, said pin and plate being adapted by engagement with the notches to hold the parts in a predetermined position relative to each other and being movable laterally to positions of disengagement from the notches on the one part to permit relative movement of the parts, and repositionable relative to each other to engage said pin and plate with other notches, means interconnecting the pins and plate comprising an actuator fixed to the plate with an end disposed in engagement with the pin such that lateral movement of the pin in the notches moves the plate laterally and spring means for coincidentally engaging said pin and plate with said other notches. 

1. A jack stand comprising a rigid hollow column adapted to be supported at one end in an upright position, a post telescopically mounted at the upper end of the column for extension relative thereto, said column containing inclined guideways below its upper end, a first part supported in an operative position by the guideways, said first part being adapted to be displaced laterally in said guideways to permit extension of the post relative to the first part for reengagement of another notch therewith, said first part being adapted to be returned to its undisplaced position when confronted by another notched portion of the post, a second part supported at the top of the column on its upper end, said second part being adapted in its operative position to have engagement with a notch at the top of the column when a notch lower down is engaged with the first part and to be displaced in an operative position of disengagement from the notch at the top of the post when the post is extended, means operable by lateral displacement of the first part to effect a coincidental lateral displacement of the second part, and spring means associated with said last means operable to return said first and second parts to their operative positions.
 2. A jack stand comprising a hollow column adapted to be supported at one end in an upright position, a post telescopically mounted at the upper end of the column for extension relative thereto, said post containing at one side a plurality of notches spaced lengthwise thereof, transversely spaced inclined guideways on the column below the top, a bar mounted in the guideways transversely of the notched side of the post with whicH the notches in the post are adapted to be engaged to support the post in a predetermined position of extension, means on the post between notches operable by extension of the post to displace the bar laterally from the notch with which it is engaged, said bar being adapted to return to its undisplaced position when confronted by a succeeding notch, a plate mounted at the top of the column adapted to be engaged with a notch at the top of the column when a notch lower down is engaged with the bar, said plate being movable laterally from a position of engagement to a withdrawn position such as to permit the post to be moved relative to the column, means interconnecting the plate and bar operable by movement of the bar to a lateral position to effect concomitant lateral movement of the plate to said withdrawn position, and spring means associated with the last means for returning the plate and bar to positions of engagement with the notches.
 3. A jack stand according to claim 2, wherein there is means supporting the plate at the top of the column comprising a lug fixed to the underside of the plate containing a hole, a pin fixed at one end to the column and extending horizontally therefrom through said hole, and a spring mounted on the pin with its ends compressably engaged, respectively, with the column and with the lug.
 4. A jack stand according to claim 2, wherein the plate rests on top of the column, a lug is fixed to the underside of the plate having spaced, downwardly extending arms containing aligned openings, a pin is fixed at one end to the column and extends horizontally therefrom through said openings, one of the arms being longer than the other and extending downwardly into adjacency with said pin, the openings in the longer arm being larger than that in the shorter arm and a spring mounted on the pin, said pin extending through the opening in the longer arm and being compressably engaged at its ends with the column and the shorter arm.
 5. A jack stand comprising a rigid hollow column adapted to be supported at one end in an upright position, a post telescopically mounted at the upper end of the column for extension relative thereto, said post containing along one side lengthwise thereof a plurality of spaced notches, transversely spaced guideways on the column below the top, a pin mounted in said guideways transversely of the notched side of the post, said pin at the lower ends of the guideways being adapted to engage a notch in the post and by such engagement to support the post in a predetermined position of extension and when moved to a lateral position at the upper ends of the guideways to become disengaged from the notches to permit the post to be extended, means on the post between notches operable when the post is extended to move the pin to said lateral position in said guideways, said pin being adapted to be returned to the lower ends of the guideways when confronted by a succeeding notch, a flat plate mounted on the top of the column having an operable position in which it engages a notch at the top of the column when a notch lower down is engaged with said pin, and an inoperative withdrawn position when the pin is moved to said lateral position, an arm fixed to the plate having a part extending downwardly therefrom to a position adjacent the pin, said part being movable by the pin when the latter is moved laterally to effect simultaneous lateral movement of the plate, and a spring engaged with the arm operable to return the plate to its operative position and simultaneously to return the pin to its operative position.
 6. A jack stand comprising a rigid hollow column of rectangular cross-section adapted to be supported in an upright position, a telescopically mounted post of rectangular cross-section situated at the upper end of the column for extension relative thereto, said post being of a size such that three sides slidingly engage three sides of the column and the fourth side is spaced from and parallel to the fourth side of the column, sAid post containing lengthwise thereof a plurality of spaced notches, a first part supported by the column below the top thereof with which a notch in the post is adapted to be engaged to support the post in an extended position as determined by one of the notches engaged with the first part, a second part supported at the top of the column arranged to engage a notch in the post above the one notch, said second part being operated to hold the post extended in the event the first part should fail, and a guide member at the lower end of the post for slidably engaging all four sides of the column.
 7. A jack stand according to claim 6, comprising a saddle plate mounted at the top of the post, said saddle plate containing a depression in its upper side and a corresponding protrusion on its lower side of a size to fit into the open top of the post, and means connecting the saddle plate to the post which permits rocking of the saddle plate about right angularly disposed axes perpendicular to the vertical axis of the post.
 8. A jack stand according to claim 6, wherein one side of the post is spaced from and parallel to the corresponding side of the column and wherein the notches are situated along said one side and each notch inclines downwardly with respect to the perpendicular axis of the post and has at its upper end a re-entrant recess of a configuration corresponding to the cross-section of said part.
 9. A jack stand according to claim 6, wherein said post has at its upper end aligned holes which are vertically elongate, comprising a saddle plate having spaced parallel, downwardly extending flanges containing aligned holes mounted on the post with the holes in the flanges in alignment with the holes in the post, and a pin mounted in said holes.
 10. A jack stand according to claim 9, wherein the saddle plate is provided with spaced parallel, upwardly divergent flanges along the edges intermediate the downwardly extending flanges.
 11. A jack stand according to claim 6, provided with a base member at the lower end of the column comprising a flat plate of larger area than the cross-section of the column on which the lower end of the column rests, spaced parallel flanges rising from the plate in engagement with the opposite sides of the column at its lower end, said flanges and the lower end of the column containing openings for receiving a bolt, a bolt disposed in said opening, said bolt being adapted to lock the column in a perpendicular position with its lower end engaged with the plate, and said openings in the column being elongate so as to permit the column to be elevated with respect to the plate when the bolt is loose to enable rotating the column to a prone position, and means on the plate situated between the flanges in a position to enter the lower end of the column when the latter is situated in a perpendicular position operable to resist displacement of the lower end of the column so long as the lower end is held engaged with the plate.
 12. A jack stand comprising slidingly interengaged parts, one of which is adapted to be supported at one end in an upright position and the other of which is adapted to be moved relative to the other end of the one part to effect extension or retraction, and means for locking the parts in a predetermined position relative to each other, comprising longitudinally spaced notches on one part, a pin mounted in inclined slots in the other part and a flat plate mounted on the other part above the pin at a distance therefrom corresponding to the distance between said notches, said pin and plate being adapted by engagement with the notches to hold the parts in a predetermined position relative to each other and being movable laterally to positions of disengagement from the notches on the one part to permit relative movement of the parts, and repositionable relative to each other to engage said pin and plate with other notches, means interconnecting the pins and plate comprising an actuator fixed to the plate with an end disposeD in engagement with the pin such that lateral movement of the pin in the notches moves the plate laterally and spring means for coincidentally engaging said pin and plate with said other notches. 